The time of arrival and location of an object at a manufacturing station is critical in certain manufacturing processes. This is particularly the case in the fabrication of integrated electrical circuits from wafers of silicon. In the past, the detection of this arrival at a station has been accomplished by the use of a photoelectric system at the station, and wherein unmodulated signal light is directed onto and is reflected from a wafer, and the detection of the reflected light signals the arrival of the wafer. Such a system functions well in environmental darkness, but does not in the presence of other typical light conditions. Thus, sunlight tends to wash out the unmodulated signal light or to be otherwise undetectable from it, and artificial light presents a distinct modulation at a 120 Hz rate which makes it difficult to discriminate between it and signal light. Further, it is often inconvenient or impractical to simply mount a light above a station and detect the obstruction of a light path because of space or other restrictions.
It is the object of this invention to provide a photoelectric system for the described usage which is relatively immune to ambient light, whether it be natural or artificially produced, will fit and operate in most situations, and is inexpensive to manufacture and install.